Date: Wed, 17 Jan 2007 01:55:09 +0100 From: "Attilio Rao" <attilio@freebsd.org> To: "Maxim Sobolev" <sobomax@freebsd.org> Cc: freebsd-current@freebsd.org, Ivan Voras <ivoras@fer.hr>, freebsd-arch@freebsd.org Subject: Re: [PATCH] Mantaining turnstile aligned to 128 bytes in i386 CPUs Message-ID: <3bbf2fe10701161655p5e686b52n7340b3100ecfab93@mail.gmail.com> In-Reply-To: <45AD6DFA.6030808@FreeBSD.org> References: <3bbf2fe10607250813w8ff9e34pc505bf290e71758@mail.gmail.com> <3bbf2fe10607281004o6727e976h19ee7e054876f914@mail.gmail.com> <3bbf2fe10701160851r79b04464m2cbdbb7f644b22b6@mail.gmail.com> <20070116154258.568e1aaf@pleiades.nextvenue.com> <b1fa29170701161355lc021b90o35fa5f9acb5749d@mail.gmail.com> <eoji7s$cit$2@sea.gmane.org> <b1fa29170701161425n7bcfe1e5m1b8c671caf3758db@mail.gmail.com> <eojlnb$qje$1@sea.gmane.org> <3bbf2fe10701161525j6ad9292y93502b8df0f67aa9@mail.gmail.com> <45AD6DFA.6030808@FreeBSD.org>
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2007/1/17, Maxim Sobolev <sobomax@freebsd.org>: > Attilio Rao wrote: > > 2007/1/17, Ivan Voras <ivoras@fer.hr>: > >> Kip Macy wrote: > >> > On 1/16/07, Ivan Voras <ivoras@fer.hr> wrote: > >> >> But it does seem to hurt the performance a bit - maybe it's time to > >> add > >> >> another CPU option like I586_CPU and I686_CPU? > >> > > >> > Unless there is a compelling reason not to do so, I think that that > >> > would be a good idea. > >> > >> Maybe even someone finds a way to get optimized versions of memcpy in > >> the kernel :) > >> > >> I was thinking: AFAIK the only major stopper is context saving of the > >> various "auxiliary" registers - FPU, MMX, SSE, right? But is it an > >> all-or-nothing situation? I.e. does it make sense (can it be done?) to > >> just elect to save the MMX context? (AFAIK they are different registers > >> than SSE, but overlay FPU registers?) The idea is to save something > >> smaller than the full set. > > > > When I implemented fpu copy into the kernel I had a lot of thinking > > about this and I think it is possible at least with some restrictions. > > For example, for an xmm copy you would just save 8 registers content > > but you have to ensure no pending FPU exceptions will break your > > kernel and so you should preserve a clean copy of FPU state or, treact > > the corner cases you can get. > > For xmm, after some very productive discussions with bde@, we arrived > > at the conclusion that should be pretty safe to just have an 16 byte > > aligned buffer for registers saving (in this way you can use 8 movdqa > > for saving them) but I didn't end to play with it. > > (My implementation should deal with the problem of pinning the > > scheduler too, in order to avoid a wrong reading of per-cpu datas). > > I might be wrong, but I think the DragonFly has solved this issue (i.e. > optimized memcpy in the kernel) somehow quite some time ago. Dragonfly saves the whole context (xmm + mmx + fpu state). It is a too heavy mechanism ATM for us (and for them too I suspect). The don't need to deal with pinning too, BTW. Attilio -- Peace can only be achieved by understanding - A. Einstein
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